The present invention is directed generally to anti-glare devices, and more particularly, to a filter for transmission screens or liquid crystal displays (LCD) for eliminating glare caused by ambient light, including horizontal light rays from behind the operator, while at the same time increasing the vertical viewing area.
It has long been recognized that the front face of a transmission screen such as the cathode ray tube (CRT) of an ordinary television screen or the CRT of a computer terminal produces glare caused by reflected ambient light. As a result of this phenomena, a substantial amount of glare is encountered when viewing, for example, a CRT in a brightly lit room. The glare problem can be reduced by increasing the intensity of the radiation from the CRT. However, manufacturing CRT's capable of producing the intensity necessary to overcome glare encountered in a brightly lit room greatly increases the cost of the CRT. It is therefore desirable to produce a filter or overlay which will reduce the glare produced by ambient light.
U.S. Pat. Application Ser. No. 716,237, filed Aug. 20, 1976, now abandoned, was directed to an overlay or filter for the viewing surface of a television screen to reduce front face glare. The overlay is constructed of a sheet of transparent material having a planar back face and a front face having a plurality of V-shaped grooves therein such that the overlay, when viewed from the side, has a saw-toothed profile. The overlay is flexible such that a film of oil can be applied to the back surface of the overlay and, by pressing the overlay against the viewing surface of the television screen, the overlay adheres thereto. The adherence of the overlay to the television screen together with the film of oil therebetween minimizes the optical interference between the overlay and the television screen. Unfortunately, this overlay did not produce satisfactory results. The overlay produced images, known as echo images, which interfered with the intelligence displayed on the television screen.
In U.S. Pat. No. 4,165,920, a front face glare reduction overlay including an echo reduction improvement is disclosed. The improvement involves applying a coating of opaque material to the peaks of the saw tooth forming the front surface of the overlay. It is also disclosed in the U.S. Pat. No. 4,165,920 that the horizontal portion of the saw tooth should be inclined approximately six degrees from the horizontal and the vertical portion of the saw tooth should be inclined approximately thirty degrees from the vertical. Using these angles in conjunction with the opaque material provides a front face glare reduction overlay which produces satisfactory results. However, despite the satisfactory results, it proved difficult to manufacture the peaks of the saw tooth with a sharp point. Because the peaks tended to have a round or lenticular shape, they were difficult to coat with the opaque material. Light from the transmission screen was refracted by the uncoated or partially coated peaks in a number of different vertical directions, thereby creating a further problem with echo images.
In an effort to provide an overlay or filter for the viewing surface of a transmission screen to reduce front face glare which is both inexpensive and easy to manufacture, the present inventor developed an anti-glare device which is described and illustrated in U.S. Pat. No. 4,473,277. The anti-glare device of this patent need not be applied directly to the surface of the CRT but can, in fact, be positioned a discrete distance therefrom. With the back surface free of the requirement of being tightly fitted to the transmission screen, it was discovered that the opaque material applied to the peaks of the saw tooth could be eliminated if a circular polarizer or some similar filter means was mounted to the back surface of the anti-glare device. The anti-glare device disclosed in the aforementioned patent proved to be easy to manufacture and free of echo image problems. However, the cost of the circular polarizer together with the time and expense required by the bonding step which bonded the circular polarizer to the anti-glare device increased the cost of the device.
In U.S. Pat. No. 4,697,881, the present inventor disclosed an anti-glare filter for reducing the glare of a viewing screen caused by ambient light coming from the ceiling area where most of the distorting light originates. The anti-glare filter was constructed of a substantially transparent sheet of material having first and second faces. The first face was substantially planar and the second face had a plurality of V-shaped grooves, each groove being formed by two walls. A first wall of the V-shaped groove was inclined at an angle of at least the critical angle for the material with respect to the first face, whereby ambient light entering the second face and reflecting off the first face was internally reflected.
However, an anti-glare filter constructed in accordance with the teachings of U.S. Pat. No. 4,697,881 having an angle less than 45.degree. and an index of refraction of the filter material of 1.41 or lower, will allow ambient light from behind the CRT operator to be reflected by the front face of the filter into the eyes of the operator. Thus, a reflection of the keyboard and the operator's moving hands may seriously interfere with the desired picture and be most disconcerting to the operator. Another problem encountered with filter material having a low index of refraction, for example 1.35, and a critical angle of 47.8.degree. (.theta.), limits the viewing area above the horizontal to 42.2.degree. (90.degree.-.theta.), since the upper limit of the viewing area is determined by the extension of the sloping side of the groove.